Selectivity control circuits



June 24, mm F, O DENO H 2,247,155

SELECTI VITY CONTROL CIRCUITS Filed June 4, 1940 2 Sheets-Sheet 1 r I n Wu m MMM BAND/245$ Z1 7'050l/R6EOF) T V SIG/VAL ENERGY INVEN TOR. ERN? F. GOODENOl/GH ATTORNEY.

E. F. GOODENOUGH ,247,155

June 4, H941.

SELECTIVITY CONTROL CIRCUITS 2 Sheets-Sheet 2 Filed June 4, 1940 70 UT/UZA TION NETWORK TOE/6511M ENERGY VS W 800m: l R k .L W7 v msou/acs OFAVC 51,45

K 70 (IT/UZAT/O/V NETWORK 7'0 SIG/VA L ENERGY SOURCE 7'0 SIG/VAL 7'0 l/T/L IZAT/ON ENERGY NETWORK SOURCE INVEN TOR. ERA/EST F GOODENOUGH ATTORNEY.

Patented June 24, 1941 were SS EN SELECTIVITY CONTRQL (JERCUITS Application .lsune 4, 1940, Serial No. 338,723 In Great Britain March 11, 1933 2 Claims.

The present invention relates to circuit arrangements for automatically varying the selectivity of tuned filter circuits, particularly such tuned filter circuits as are used in radio and like receivers.

Automatic selectivity devices in radio receivers are of course well known, such devices being arranged so that the selectivity of the receiver is inversely proportional to the strength of received signal. In one such device, means responsive to signal strength is provided to vary the effective resistance of one of a plurality of coupled tuned circuits, and also to vary the effective coupling of the said tuned circuit. The variation in effective resistance of the primary of the tuned circuit is effected by applying to a grid of the valve feeding the tuned circuit, a voltage displaced by 180 from the anode voltage, and also feeding to the said grid a voltage displaced 90 from the signal voltage applied to the control grid of the said valve. Both these voltages are derived from devices coupled to the tuned circuit primary or secondary, the device for deriving the 90 phase-shifted voltage including a phase-shifting network.

According to the present invention, in its widest aspect, a network comprising a filter including a pair of tuned circuits coupled by a reactance and an impedance coupled together by a valve is provided with a feedback circuit including a reactive coupling between the impedance and that end of the filter which is remote from the said impedance.

The impedance may be one tuned circuit of a further pair of tuned circuits coupled by a reactance, and according to the invention in this aspect, a network comprising two filters each including a pair of tuned circuits coupled by a reactance and coupled together by means of a valve is provided with a feedback circuit including a reactive coupling between the inputs of the two filters or between the outputs of the two filters.

In order that the pass band of the system may be narrowed as gain of the coupling valve is increased, the feedback reactance has the same sign as the coupling in that filter across which and the valve, the feedback circuit extends. If desired, a negative feedback may be provided, for example, by including in the cathode lead of the valve a suitable negative feedback network. This has the effect of rendering the arrangement more stable, and of spreading the range of control of selectivity over a larger portion of the automatic volume control (A. V. C.) range of the valve. An

improvement in the frequency characteristic can also be obtained by this means.

In the drawings:

Fig. 1 schematically shows the basic circuit of the invention,

Fig. 2 shows a modification,

Figs. 3a, 3b, 3c show respectively different types of band pass filters which may be employed in the circuit of Fig. 1,

Figs. 4 and 5 show two forms of degenerative feedback circuits that may be added to tube V,

Fig. 6 shows an amplifier circuit employing a form of the invention,

Figs. 7a. and '7 b show two different modifications of the circuit of Fig. 6.

In the circuit arrangement of Fig. 1 a two section coupled band pass filter, represented generally at Fl, is fed with signal energy, and has its output connected between grid and cathode of a valve V. The valve feeds into an impedance 2! which may be the input impedance of another band pass coupled filter. The coupling of the two section filter consists of a negative reactance. and may be effected by using negative mutual inductive coupling (-M) as illustrated in Fig. 3a, or by using series capacity (that is, the coils are connected in the manner known as series-aiding) coupling as in Fig. 3b, or there may be used shunt capacity coupling as in Fig. 3.0. A small feedback condenser C is connected between the input of the two section band pass filter Fl which feeds into the Valve V and the impedance into which the valve feeds. The condenser C thus shunts the valve V and the two section filter Fl.

Alternatively, and as illustrated in Fig. 2, the impedance, which in this case may be the output impedance of a further band pass filter, is connected to the valve V and the valve feeds into a two section filter the sections of which, as before, are coupled by a negative reactance. In this modification, the small condenser C is connected between the said impedance and the output of the two section filter into which the valve V feeds. The valve should preferably be of the variable mu type, and may be either a tetrode or a pentode, the mutual conductance then being variable either automatically by the usual A. V. (3. voltage, or manually, or both.

An improvement in ease of control and in stability may be provided by the use, in addition, of negative feedback applied to the control valve. The preferred method, illustrated in Figs. 4 and 5, is to connect an impedance in the cathode lead. This, as shown in Fig. 4, may take the form of a resistance R connected in the cathode lead of valve V, the resistor being shunted by a parallel resonant circuit LC to provide a direct current path of low resistance.

An improvement in cut-off may be provided by including, as shown in Fig. 5, in the cathode lead of the valve V two resonant circuits LCI and LCZ; one is tuned slightly above and one slightly below the mid-band frequency, the two circuits being connected in series.

Another advantage of the use of negative feedback, in addition to positive, is that the range of control of the valve is increased; that is to say, a bigger change of a (mutual conductance) is required to vary the band width from its broadest to its narrowest limits. Hence, in practice, the band width is varied over a wider range of A. V. C. voltage with the use of negative feedback than without.

The adjustment of the filter is performed in the ordinary way. The negative bias on the control grid of the valve is made large to reduce the feedback, and the filters respectively feeding into, and fed by, the valve are tuned to resonance. The response should then be the usual broad pass band. On reducing the bias, the gain increases, and the pass band narrows to a sharp single peak.

Fig. 6 illustrates a circuit arrangement in which two cascaded band pass filters F! and F2, each including a pair of tuned circuits coupled by negative mutual inductance l\/I, are coupled together by means of valve V, the input of F2 being back coupled to the input of Fl by means of feedback condenser C. Alternatively,.if desired, the output of F2 may be back coupled to the output of Fl by condenser C. For the sake of simplicity of description, all direct current energizing sources, as well as self-bias resistors, for the amplifiers are omitted. Those skilled in the art are fully acquainted with the nature of such omitted circuits.

The operation of the circuit may be briefly explained as follows, with the arrangement of Fig. 6 in mind:

The voltage as appearing at the output 'terininals of the filter Fl leads the input current to by 90 when the coupling reactance is negative. The input impedance of the following filter F2, when in tune, is a pure resistance. On account of the phase reversal occurring in the valve, the

voltage ca at the anode of V therefore leads '0 by 90.

In practice, where the stage gain is considerable, the value of the feedback capacity C required is very small. Its impedance is, therefore, very large compared with the sum of the input impedances of the two filters, and it may be assumed that the feedback current i is directly proportional to C and to the voltage 12a at the anode of the valve. The current through a condenser leads the voltage across it by 90". Therefore, the lag of ca on i0 is corrected by the lead of ic on va, with the result that is is in phase with This constitutes positive feedback, and the output at the mid-band frequency is increased to an extent depending on the stage gain, i. e. on g. Moreover, the relative phase change of the v'olt ages at the input and output of the respective filters is such that this increase persists over a band of frequencies in the neighborhood of the in-tune frequency, the width of this band being smaller the greater the value of g. Thus, increasing g has the effect of increasing the output and narrowing the pass band width at the same time, when the mutual coupling of the preceding filter across which the feedback condenser is connected is negative.

As regards the other succeeding filter whose input impedance is all that matters, the type of coupling may be +e or -e mutual inductance, or capacity, so long as it has the right impedance characteristic with respect to the equivalent impedance (i. e. output voltage per input current) characteristic of the preceding filter over which feedback occurs. Also, since a negative mutual is required in the latter filter, capacitative coupling may be used in it in any convenient form (series or shunt coupling) and the correct phase conditions for narrowing the band with increase of y will be obtained. If a coupling of the opposite sign be used, the pass band will broaden as g is increased. The source of A. V. C. bias has not been shown, since those skilled in the art are fully acquainted with the nature thereof. In general, any arrangement may be used which derives a direct current voltage from the signal carrier, the magnitude of the direct voltage being proportional to the carrier amplitude.

In the case where the feedback condenser couples the outputs of the two filters, the action is similar to the above except that the actions of the two filters are interchanged. Thus the phase change (and hence the sign of the coupling) in the following filter becomes important, While the preceding filter supplies the appropriate input impedance across which the fed back voltage is developed.

Figs. 7a and 7b show alternative arrangements in which the condenser C of Fig. 6 is replaced by the condenser C. The value of C of Figs. 7a and 7b is given approximately by where C1 and C2 are the capacities of the condensers so indicated in Figs. 7a. and 7b; and C is the capacity of the condenser so marked in Fig. 6.

Instead of using a capacitative back coupling element in the feedback circuit, a small mutual inductive back coupling may be used between either the two input circuits (respectively of filters F! and F2) or the two output circuits. In this case the coupling M may be negative or positive depending upon the sign of the small mutual back coupling, the requisite condition being that they be of the same signs.

What is claimed is:

1. In a signal transmission system, a first filter network comprising a pair of signal-tuned circuits reactively coupled, a second filter network comprising a second pair of reactively coupled signal-tuned circuits, an electron discharge tube coupling the second filter in cascade with the first filter, at least one tuned circuit of said first pair including a condenser, a second condenser in series with said first condenser, at least one tuned circuit of said second pair of circuits including a third condenser, said second condenser being connected in series with the third condenser, said second condenser having a terminal at a fixed alternating potential and providing regenerative feedback of signal voltage from said second filter network, and means for varying the gain of said tube to vary the degree of feedback.

2. In a signal transmission system, a first filter network comprising a pair of signal-tuned circuits reactively coupled, a second filter network denser, said second condenser having a terminal 10 at a fixed alternating potential and providing regenerative feedback of signal voltage from said second filter network, and means for varying the gain of said tube to vary the degree of feedback, the reactive coupling betwen the tuned circuits of each pair being mutual inductance, and said varying means being responsive to signal amplitude variation.

ERNEST FREDERICK GOODENOUGH. 

